There is known a valve device for an exhaust flow passage, which changes a flow passage mode by opening and closing an exhaust flow passage in an exhaust system for an internal combustion engine. Patent Document 1, for example, discloses a valve device for an exhaust flow passage, which opens and closes an opening formed in a separator that divides inside of a muffler for an internal combustion engine into an upstream chamber and a downstream chamber. In this valve device for an exhaust flow passage, a valve body capable of closing the opening is supported by a support body so as to be rotationally movable; the valve body is biased by a coil spring in a valve closed direction. Thus, when a number of revolutions of the internal combustion engine is low, a pressure of exhaust gas in the upstream chamber is low, which leads to a valve closed state that is a state of the opening being closed by the valve body. As a result, exhaust noise can be reduced. In contrast, when the number of revolutions of the internal combustion engine increases, a pressure of exhaust gas in the upstream chamber becomes high, which causes the valve body to be away from the opening, resulting in a valve open state that is a state of the opening being open. As a result, pressure loss can be reduced.
However, in the configuration described in Patent Document 1, as the valve body moves towards a valve open direction, the biasing force of the coil spring becomes larger. For this reason, if the coil spring is designed to provide a larger biasing force by focusing on exhaust noise reduction in the low revolution state, an effect of reducing pressure loss in the high revolution state may be impaired. In contrast, if the coil spring is designed to have a smaller biasing force by focusing on pressure loss reduction in the high revolution state, an effect of reducing exhaust noise in the low revolution state may be impaired.
In view of the above, for example, as described in Patent Document 2, a valve device for an exhaust flow passage, which includes a toggle linkage mechanism, has been proposed. The valve device for an exhaust flow passage, which includes the toggle mechanism, comprises a support-side link member supported by a support body so as to be rotationally movable, and a valve-side link member supported by a valve body so as to be rotationally movable and coupled to a support-side link member so as to be rotationally movable with respect to each other. That is, the toggle mechanism is formed of the support body, the valve body, the support-side link member, and the valve-side link member. Therefore, in a valve closed state, as a link angle that is formed by the support-side link member and the valve-side link member is larger (i.e., closer to 180 degrees), a stronger external force is required to rotationally move the valve body towards a valve open direction. Accordingly, compared with the configuration in which the valve body is maintained in a valve closed state only with a biasing force of a biasing member (for example, the aforementioned configuration described in Patent Document 1), it is possible to increase the external force necessary to open the valve body, while reducing the biasing force of the biasing member.
In order to enhance a closed-valve maintaining force of the toggle mechanism, it is necessary to design the link angle to be larger in the valve closed state; however, when the link angle reaches 180 degrees, a malfunction may be caused in which the valve body cannot be opened normally. Thus, in the configuration described in Patent Document 2, a stopper mechanism is provided to limit the link angle to be smaller than 180 degrees upon abutment of a support-side stopper portion formed on the support-side link member against a valve-side stopper portion formed on the valve-side link member.